This invention relates to a semiconductor light emitting element and its manufacturing method. More particularly, the invention relates to a semiconductor light emitting element and its manufacturing method preventing thermal deterioration of nitride compound layer containing indium and deterioration of interfaces, and thereby promising growth of a high-quality nitride compound semiconductor light emitting element, by restricting materials of layers overlying the nitride compound layer containing indium to specific materials or by restricting growth temperatures within a predetermined range upon stacking a plurality of semiconductor layers involving the nitride compound layer containing indium on a substrate.
Most of nitride compound semiconductors are optically direct-transitional and capable for highly efficient radiative recombination. Their bandgap energy widely ranges from 1.89 to 6.2 eV. For these favorable natures, development is under progress for using them as high-efficient light emitting elements, such as various kinds of short-wavelength semiconductor lasers and high-luminous visible LEDs.
In the present application, the term xe2x80x9cnitride compoundxe2x80x9d pertains to any compound semiconductor which can be expressed by the chemical formula BxInyAlzG1-x-y-zN (0xe2x89xa6x less than 1, 0xe2x89xa6yxe2x89xa61, 0xe2x89xa6zxe2x89xa61, x+y+zxe2x89xa61) with any values of the mole fractions x, y and z in their respective ranges. For example, InAlN (x=0, y=0.4, z=0.6) is also regarded as one of nitride compound semiconductors.
Nitride compound semiconductors can be expressed as combinations of gallium nitride, aluminum nitride, indium nitride and boron nitride which are binary semiconductors. Among them, gallium nitride (GaN) has been a subject of active developments. Gallium nitride has a melting point as high as 1700xc2x0 C., the equilibrium vapor pressure of nitrogen under a growth temperature is very high. Therefore, it is difficult to grow gallium nitride in bulk single crystal, and its crystalline growth mainly relies on hydride chemical vapor deposition (HCVD) or metal organic chemical vapor deposition (MOCVD). Among them, crystal growth technique by MOCVD has been most actively developed, and has succeeded in growing ternary mixed crystals, such as gallium indium nitride (GaInN) made by adding indium to gallium nitride, gallium aluminum nitride (GaAlN) made by adding aluminum to gallium nitride, and indium aluminum nitride (InAlN) made by adding aluminum to indium nitride.
By utilizing a heterojunction of these materials, light emitting efficiency can be improved. When using a double hetero structure effective for confinement of injected carriers or light, highly luminous LED or shortwave-length semiconductor lasers can be realized.
As to gallium indium nitride which is a ternary mixed crystal, its band gap energy can be changed from 3.4 eV of gallium nitride (GaN) to 1.89 eV of indium nitride (InN) by changing the mole fraction of indium, and it can be used as active layers of light emitting elements over wide visible wavelength bands.
Gallium indium nitride can be made by combining gallium nitride and indium nitride. However, gallium nitride needs a growth temperature of 1000xc2x0 C. or more to ensure an acceptable crystallographic quality whereas indium nitride must be grown under a lower temperature because of a high vapor pressure of indium.
These growth temperatures are taught in greater detail, for example, in Appl. Phys. Lett., 59, (1991) p2251. As taught there, the growth temperature for crystalline growth of gallium indium nitride containing a relatively higher rate of indium must be lower than that of gallium nitride.
As to the growth temperature of gallium aluminum nitride, there is a teaching in Appl. Phys. Lett., 64, (1994) p1535. As taught there, a temperature as high as that for gallium nitride is preferable for the epitaxial growth of gallium aluminum nitride.
As reviewed above, the optimum temperature is different for individual compound semiconductors. Therefore, in order to grow a double-heterostructure sandwiching an active layer made of gallium indium nitride between cladding layers made of gallium nitride or gallium aluminum nitride, for example, the growth temperature must be changed at the interfaces.
However, if the layers are grown in this mode, indium having a high vapor pressure evaporates from the surface of the gallium indium nitride layer already grown, when the temperature is raised after the growth of the gallium indium nitride active layer. It results in deteriorating the crystallinity of the gallium indium nitride layer and its interfaces with the cladding layers, and hence results in deterioration of operative characteristics of the device.
It would be a possible approach for preventing evaporation of indium to decrease the composition ratio of indium. However, composition of indium is an essential factor which determines band gaps and other basic properties, and any change in its value may invite undesirable changes in essential characteristics such as emission wavelength.
It is therefore an object of the invention to provide a semiconductor light emitting element and its manufacturing method permitting a hetero structure to be made without degrading the crystalline quality of nitride compound layer containing indium, such as gallium indium nitride, having a predetermined composition ratio of indium.
According to the invention, there is provided a method for manufacturing a semiconductor light emitting element having a first layer of a first nitride compound semiconductor containing indium and a second layer stacked on the first layer, comprising the step of:
stacking the second layer under conditions inside the closed region defined by connecting points plotted at x and y coordinates (364, 600), (364, 1010), (550, 1010), (650, 600) and (364, 600) on a graph taking emission wavelengths based on band-to-band transition of said first layer in nanometer on the x axis and taking growth temperatures of said second layer in xc2x0 C. on the y axis. In this manner, a high-performance light emitting element can be made without inviting thermal deterioration of the first layer.
When using MOCVD for stacking the layers and stacking a cap layer made of a nitride compound semiconductor having a low mole fraction of indium to cap the first layer, a layer above the cap layer can be stacked under conditions outside the closed region on the graph without inviting thermal deterioration of the first layer.
The first layer may be made of gallium indium nitride, and the cap layer may be made of gallium aluminum nitride, and the layer above the cap layer may be made of nitride compound.
The first layer and the second layer preferably have thicknesses not exceeding critical thicknesses in terms of generation of crystallographic defects caused by lattice mismatching, and the gallium indium nitride stacked as the first nitride compound semiconductor may have a mole fraction within the miscibility gap under an operation of a distortion energy by lattice mismatching.
According to the invention, there is further provided a semiconductor light emitting element including a substrate and a multi-layered structure of nitride compound semiconductors stacked on the substrate, wherein the multi-layered structure includes at least a first layer of gallium indium nitride, a second layer of gallium aluminum nitride stacked on said first layer, and a third layer of gallium nitride stacked on said second layer, the first and second layers having thicknesses not exceeding critical thicknesses in terms of generation of crystallographic defects caused by lattice mismatching, and the gallium indium nitride forming the first layer having a mole fraction within its miscibility gap.
According to the invention, there is also provided a semiconductor light emitting element having a substrate and a multi-layered structure of gallium nitride compound semiconductors stacked on the substrate, wherein the multi-layered structure includes at least an active layer of gallium indium nitride, and a cladding layer of gallium indium nitride stacked on the active layer, the active layer having a mole fraction of indium higher than that in the cladding layer. In this manner, thermal deterioration of the active layer is suppressed, and the contact resistance of the electrode can be reduced.
Even when the cladding layer contains an acceptor impurity, inactivation of the acceptor impurity is less liable to occur.
The cladding layer may include two layers which are different in composition to improve the emission efficiency and the contact characteristics of the electrode.
The cladding layer overlying the active layer can be stacked at a higher temperature than the stacking temperature of the active layer during growth of the light emitting element to ensure growth of a higher-quality crystal.
The invention is used practically in the above-summarized modes, and attains various effects shown below.
Since conditions in the optimum range are used for growing layers overlying the gallium indium nitride layer after growth thereof, the multi-layered structure including the gallium indium nitride layer can be made with a high crystalline quality minimizing thermal deterioration of the gallium indium nitride layer.
As a result, various characteristics of the semiconductor light emitting element using gallium nitride compound semiconductors, including its emission efficiency, can be improved and stabilized.
In the present invention, the cap layer is grown on the gallium indium nitride layer under conditions selected to prevent thermal deterioration. Therefore, even when the temperature is raised thereafter, no thermal deterioration occurs in the gallium indium nitride layer. As a result, all of the layers of the multi-layered structure of gallium nitride compound semiconductors have high crystallographic properties.
The invention can prevent thermal deterioration of the gallium indium nitride layer more effectively by advantageously utilizing mis-fit distortion caused by a difference in lattice constant.
By more positively utilizing mis-fit distortion caused by a difference in lattice constant, the invention can grow mixed crystals with mole fractions in the miscibility gap, which have been difficult to grow heretofore. Therefore, the range of application of nitride compound semiconductors can be expanded remarkably.
The invention can prevent thermal deterioration of the active layer in the nitride compound semiconductor light emitting element, and can make interfaces satisfying both steepness and smoothness by using gallium indium nitride as the material of the p-type cladding/contact layer. It also contributes to reducing the contact resistance with the electrode, and attains epoch-making improvements of light emitting efficiency and other various characteristics of semiconductor light emitting elements.
Additionally, inactivation of acceptors is less liable to occur. Therefore, active acceptors can be obtained without the need for specific processing such as electron beam irradiation or annealing. It results in simplifying the manufacturing process of light emitting elements, improving the yield and productivity of semiconductor elements.
Thus, the invention makes it possible to produce nitride compound semiconductors or light emitting elements with a high performance and a high reliability in a simple process with a high yield, and its industrial merits are great.